[core] #include <sys/filio.h> for FIONREAD (fixes #2726)

illumos (OpenIndiana) gets FIONREAD from <sys/filio.h>

x-ref:
  "lighttpd 1.4.40 compilation fails on illumos (OpenIndiana)"
  https://redmine.lighttpd.net/issues/2735

src/sys-socket.h

@@ -21,6 +21,11 @@
 #include <arpa/inet.h>
 
 #include <netdb.h>
+
+#ifdef HAVE_SYS_FILIO_H
+#include <sys/filio.h>  /* FIONREAD (for illumos (OpenIndiana)) */
+#endif
+
 #endif
 
 #endif